The present disclosure relates generally to non-invasive diagnostic measurements dependent on pulse spectra and, more particularly, to photoplethysmographic measurements taken with a controlled application of pressure.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Diagnostic measurements, such as pulse oximetry and non-invasive measurements of total hemoglobin, may be determined from pulse spectrum measurements at varying wavelengths of light. For example, pulse oximetry may involve measurements at wavelengths of approximately 660 nm and 900 nm, and non-invasive measurements of total hemoglobin may involve measurements of wavelengths of approximately 1320 nm and 800-900 nm. In operation, conventional two-wavelength photoplethysmographic sensors may emit light from one or more emitters (e.g., light emitting diodes (LEDs) or fiber optic cables to one or more remote light sources) into a pulsatile tissue bed and collect the transmitted light with a detector (e.g., a photodiode or fiber optic cables to a remote photodetector). The detected light may then be utilized to estimate, for example, a level of oxygen saturation in the blood that is present in the tissue bed. The emitters and detector may be positioned in various orientations. In a transmission-type photoplethysmographic sensor, the emitters and detector are positioned substantially opposite one another (e.g., on opposite sides of a patient's finger), while in a reflectance-type photoplethysmographic sensor, the emitters and detector are placed adjacent to one another.
Signals from a photodetector of a photoplethysmographic sensor may be decoded to ascertain a plethysmographic waveform, which may be due to the cycling light attenuation caused by the varying amount of arterial blood that the light from the emitters passes through. Various factors may cause diminished signal quality or cause inconsistent or unreliable plethysmographic waveform readings. Specifically, the presence of excessive extravascular fluid or venous blood in a tissue bed of interest may interfere with the detection of arterial blood, producing inaccurate or inconsistent plethysmographic waveforms. The quantity of extravascular fluid or venous blood in a tissue bed of interest may vary from patient to patient or from time to time for the same patient.